1. Field of the Invention
The present invention relates to the operation of electrochemical power cells, and in particular, a system and method for passive management of fluid reactants and byproducts within a liquid feed fuel cell, such as a direct methanol fuel cell.
2. Brief Description of the Related Art
Electrochemical power cells are well suited for a variety of applications by virtue of their efficiency, environmentally friendly nature and high power densities. There are three significant types of electrochemical power sources: primary cells, secondary cells and fuel cells. Unlike primary and secondary cells, where the reactants and products are contained within the cell, fuel cells employ reactants, which are continuously supplied to the cell; byproducts are also continuously removed. Therefore, fuel cells need an efficient fluids (i.e., reactants and byproducts) management system.
One example of a fuel cell is a direct methanol fuel cell (DMFC). The DMFC has emerged as an attractive power source for portable devices because of its high energy density in generating electrical power from fuel. DMFC systems may be divided into active and passive. Active systems use a pump and fan to feed fluids, such as methanol (CH3OH) and oxygen (O2), into a cell stack, where the oxygen reacts with the methanol to produce electricity, such as fuel cells disclosed in U.S. Pat. Nos. 6,727,016 and 6,696,189, which are hereby incorporated herein by reference. Active systems are more complicated and consume electrical power from the stack: these are better suited for larger fuel cells. However, passive DMFCs have a simpler structure that requires no pump or fan, and that uses passive methods to deliver and circulate methanol and oxygen in the cell stack, such as fuel cells disclosed in U.S. Pat. No. 6,737,181, U.S. Pat. No. 6,632,553, U.S. Pat. No. 6,596,422 and U.S. Pat. No. 6,566,003, which are hereby incorporated herein by reference.
Currently, one of the most fundamental limitations of direct methanol fuel cells is that the fuel supplied to the anode of the DMFC must be a very dilute aqueous methanol solution (usually 0.5˜1.5 M, which is translated into a methanol mass concentration of 1.6% to 4.8%). If the methanol concentration is too high, a methanol crossover problem will occur, which can significantly reduce the efficiency of the fuel cell and considerably shorten the life of the proton conductive membrane. Conversely, if a DMFC were filled with a dilute aqueous methanol solution, the fuel cell operation time per refueling would be very short, considerably diminishing the advantage of a DMFC over a conventional battery.
To overcome this difficulty, a complex active driven DMFC system based on the modern micro system technology was developed. As illustrated in FIG. 1, a typical prior art active driven air-breathing DMFC system is composed of the following components: fuel cell stack, methanol sensor, carbon dioxide (CO2) separator, electronic controls, methanol feed pump, circulation pump and pump drivers.
An active driven fuel delivery system adds considerable cost to the fuel cell system and consumes considerable amounts of electricity from the fuel cell, which in turn significantly reduces the net power output of the fuel cell. As a result, the active driven DMFC is not competitive relative to the conventional battery technology in terms of cost and power output.
One alternative to active driven systems is to operate DMFC systems passively. Several approaches were examined for passive operation of a DMFC. The first involves using a polymer membrane with reduced methanol crossover (e.g., systems provided by Polyfuel and Samsung). The second method involves delivery of methanol fuel to the anode by certain passive ways (e.g., systems provided by MTI, Toshiba, Manhattan Scientific and Florida International University). The third involves use of a liquid electrolyte in lieu of a solid polymer membrane (e.g., systems provided by Medis).
Some of the disadvantages of these systems include the potential for an excessively high methanol concentration due to use of predetermined methanol feed rates based on fuel cell design and not on actual methanol concentration within the fuel cell, the need to recover methanol vapor mixed with carbon dioxide in the effluent prior to release in the ambient air, the temperature sensitivity of the methanol vapor feed, inflexibility, the incompatibility of the system with a wide range of fuel cell sizes, the inconsistent methanol distribution inherent in systems with free-moving liquid and the dependency on system orientation for efficient operation.
Thus, what is needed is a system and method for providing an efficient thermal-fluids management in a DMFC. In particular, a system is needed that incorporates a passive method for fluid transport in a DMFC which can supply neat methanol to the cell with uniform distribution, transport water (H2O) from the cathode (air) side to the anode (fuel) side, supply oxygen to the cell with uniform distribution across the cell, and release carbon dioxide from the cell. Additionally, a system is needed in which the methanol feed rate is based on actual methanol concentration within the DMFC.
A system is also needed in which the methanol and water are maintained in the liquid phase, but preferably not freely moving through the system, thus eliminating the methanol feed temperature constraints, operational dependency on physical orientation, and need for separation of methanol vapor from carbon dioxide. Such a system must also provide uniform fluid distribution. Any system satisfying the above needs must also be capable of use in DMFCs of various sizes.
It should be readily apparent that a system and method meeting the aforementioned needs would provide higher efficiency operation and yield advantages in fuel cell applications which may not have been contemplated due to the limitations of prior art systems and methods, including those described above.